Relating introspective accuracy to individual differences in brain structure

Abstract

The ability to introspect about self-performance is key to human subjective experience, but the neuroanatomical basis of this ability is unknown. Such accurate introspection requires discriminating correct decisions from incorrect ones, a capacity that varies substantially across individuals. We dissociated variation in introspective ability from objective performance in a simple perceptual-decision task, allowing us to determine whether this interindividual variability was associated with a distinct neural basis. We show that introspective ability is correlated with gray matter volume in the anterior prefrontal cortex, a region that shows marked evolutionary development in humans. Moreover, interindividual variation in introspective ability is also correlated with white-matter microstructure connected with this area of the prefrontal cortex. Our findings point to a focal neuroanatomical substrate for introspective ability, a substrate distinct from that supporting primary perception.

Figure 1

Behavioral task. Subjects completed a two-alternative forced choice task that required two judgments per trial: a perceptual response followed by an estimate of relative confidence in their decision. The perceptual response indicated whether the first or second temporal interval contained the higher contrast (pop-out) Gabor patch (highlighted here with a dashed circle which was not present in the actual display), which could appear at any one of 6 locations around a central fixation point. Pop-out Gabor contrast was continually adjusted using a staircase procedure to maintain ~71% performance. Confidence ratings were made using a 1-6 scale, with participants encouraged to use the whole scale from 1 = low relative confidence to 6 = high relative confidence.

Figure 2

ROC calculation and behavioural performance (A) Participants’ confidence ratings were used to construct a Type II ROC function that quantifies the ability to discriminate between correct and incorrect responses cumulated across levels of confidence. A_roc was calculated as the shaded area between the ROC curve and major diagonal (21). Mutually perpendicular dotted and solid lines represent the minor and major diagonals respectively. (B) Plot of the relationship between task performance (% correct) and A_roc, with subjects ordered by increasing A_roc.

Figure 3

Gray matter volume correlated with introspective ability (A) Projection of statistical (T) maps for positive (hot colormap) and negative (cool colormap) correlations with A_roc onto an inflated cortical surface T1-weighted template, thresholded at T > 3 for display purposes. Significant clusters (P < 0.05, corrected for multiple comparisons) where metacognitive ability correlated with gray matter volume (see Methods) were found in right anterior prefrontal cortex (Brodmann area 10; positive correlation) and left inferior temporal gyrus (negative correlation), accompanied by contralateral homologous clusters at P < 0.001, uncorrected. (B) Plot of grey matter volume in the right BA10 cluster against both A_roc and d’ (see Methods for full details) indicating that the correlation with metacognitive ability was independent of task performance.

Figure 4

White matter microstructure correlated with introspective ability (A) Statistical (T) map of voxel-wise correlations between fractional anisotropy (FA) and A_roc, thresholded at T > 3 for display purposes and overlaid on sagittal (left) and axial (right) slices of the average FA image across subjects, at the x and z co-ordinates indicated. A region within the genu of the anterior corpus callosum showed a correlation between FA and metacognitive accuracy that was statistically significant after correcting for multiple comparisons (P < 0.05). (B) Plot of FA in the anterior corpus callosum cluster against both A_roc and d’ indicating that the correlation with metacognitive ability was independent of task performance.